A most important recent finding is that activation of a complex- bound valyl-tRNA synthetase requires tRNA, and that a heat-stable regulatory protein is associated with tRNA. It appears that the oxido-reductive regulatory function of the protein's thiol groups is transmitted through an effect on tRNA conformation. The nucleic acid-protein complex is obtained from the soluble portion of a heated extract by precipitation in acid. The protein can be obtained from the complex by extraction into phenol, which is then removed by dialysis against dilute hydrochloric acid (pH 3). The acid-soluble protein becomes insoluble when neutralized but dissolves in a neutral solution of tRNA.